Electronic devices generate heat during operation. Thermal management refers to the ability to keep temperature-sensitive elements in an electronic device within a prescribed operating temperature.
Historically, electronic devices have been cooled by natural convection. The cases or packaging of the devices included strategically located openings (e.g., slots) that allow warm air to escape and cooler air to be drawn in.
The development of high performance electronic devices, such as processors, now requires more innovative thermal management. Each increase in processing speed and power generally carries a “cost” of increased heat generation such that natural convection is no longer sufficient to provide proper thermal management.
One common method of cooling electronic devices includes thermally coupling a heat sink to the package of the electronic device. A typical heat sink includes protrusions such as fins or pins that project from a body of the heat sink. The protrusions give the heat sink a larger surface area such that the heat sink dissipates a greater amount of heat from the package into the surrounding environment. Heat sinks are fabricated from materials with high thermal conductivity in order to efficiently transfer thermal energy from the electronic device package.
FIG. 1 shows a prior art electronic assembly 6. Electronic assembly 6 includes an integrated circuit such as die 8 that is secured within an integrated circuit package 14. Integrated circuit package 14 is typically soldered or plugged into a motherboard on a computer. Integrated circuit package 14 includes a heat spreader 12 that is connected to a heat sink 10. Heat sink 10 cools the integrated circuit package 14 during the operation of an electronic system that includes die 8.
A thermal interface material 16 is sometimes used to promote an effective thermal path between heat spreader 12 and heat sink 10. Thermal interface material 16 is typically in the form of a paste or tape.
New thermal interface materials with higher thermal conductivities are continually being developed to meet the requirements for more efficient heat removal. These improved materials are necessary to keep the next generation of processors operating at lower temperatures.
Some of the new thermal interface materials are phase-change materials. These new phase-change materials have been proven to be thermally superior to other types of thermal interface materials.
As used herein, a phase change thermal interface material is a material that changes from solid to liquid when its temperature is raised above a certain level. The phase transition temperature of thermal interface material 46 is below the operating temperature of the junction between heat sink 10 and integrated circuit package 14 but above ambient temperature such that there is a transition from solid to liquid. As thermal interface material 16 changes to a liquid, it flows into the cracks in heat sink 10 and heat spreader 12. When thermal interface material 16 cools below a certain temperature, it turns permanently back into a solid.
Heat sink 10 is typically compressed against heat spreader 12 by adhesives, screws, and/or bolts. Another common method uses one or more clips to compress heat sink 10 against heat spreader 12.
Compressing heat sink 10 against integrated circuit package 14 decreases the thermal impedance between integrated circuit package 14 and heat sink 10. However, the new phase-change materials are often squeezed, or squished, out from between heat sink 10 and heat spreader 12.
FIG. 2 shows a compressive force (designated by arrow A) applied to heat sink 10 and integrated circuit package 14. Thermal interface material 16 tends to squish out the sides as it changes from solid to liquid, since it is sandwiched between two flat surfaces on heat sink 10 and heat spreader 12. Larger compressive forces generate more leakage. Containing thermal interface material 16 is particularly critical when metal-based, electrically conductive, low-melting temperature alloys are used as thermal interface material 16. The escaping thermal interface material 16 forms droplets 18 that can drip off the electronic assembly 6 onto a surface of a substrate such as a printed circuit board. If the droplets 18 fall onto the substrate, they may contaminate one or more electrical pathways.
The flat mating surfaces on heat sink 10 and heat spreader 12 come into contact as heat sink 10 engages heat spreader 12. Therefore, electronic assembly 6 does not include the ability to maintain bond line thickness between heat sink 10 and heat spreader 12 or align heat sink 10 relative to heat spreader 12. Maintaining bond line thickness between heat sink 10 and heat spreader 12 would be desirable because a pre-specified volume of the thermal interface material could be contained between heat sink 10 and integrated circuit package 14.
There is a need for an electronic assembly that adequately maintains a thermal interface material between a heat sink and an integrated circuit package. In addition, any improved electronic assembly should be able to control bond line thickness between the heat sink and integrated circuit package. Interlocking the heat sink and integrated circuit at least partially together would also be desirable.